1. Field of the Invention
The invention pertains to optical systems, e.g., optical fiber communication systems and optical mass storage devices, which include thin film polarization rotators subject to essentially zero linear birefringence, as well as to methods for fabricating such rotators.
2. Art Background
Optical systems for communicating and storing information are known and are now commerically significant. For example, an optical communication system, as schematically depicted in FIG. 1, typically includes a light source, such as a semiconductor laser, which emits a light signal, e.g., an information-carrying light signal, to an optical fiber, which transmits the light signal to a photodetector. An optical mass storage device, as schematically depicted in FIG. 2, typically includes an optical disk which is capable of being, or has been, processed to store information. This information is encoded onto the disk (through processing) as regions of different optical properties, e.g., different optical reflectivity. The disk is read, i.e., the information stored on the disk is detected, by shining light from a light source, e.g., a semiconductor laser, (typically through a beam splitter) onto the disk. The light reflected from the disk is then directed (i.e., refelected by the beam splitter) to a photodetector. Alternatively, the light transmitted by the disk is directed to a photodetector.
In a wide variety of optical systems, devices that rotate the polarization of linearly polarized light in the same sense irrespective of traversal direction are advantageously included. In this regard, the frequency and power intensity spectra of the light emitted by the semiconductor lasers employed in optical systems are altered when reflected light impinges upon the lasers. Such alterations are undesirable because they lead to errors in the detected information. Thus, efforts have been made to develop devices, called optical isolators, for isolating the semiconductor lasers from reflected light. An optical isolator based on rotation of linearly polarized light is exemplified, as depicted in FIG. 3, by a bulk magnetic garnet material, e.g., bulk single crystal yttrium iron garnet (Y.sub.3 Fe.sub.5 O.sub.12, called YIG) material, positioned between a polarizer and an analyzer. This optical isolator has been proposed for use with optical fiber communication systems operating at a wavelength of about 1.3 .mu.m because single crystal YIG is substantially transparent (at least 50 percent of the incident light is transmitted) at infrared wavelengths (wavelengths ranging from about 0.8 .mu.m to about 6 .mu.m). In operation, a magnet is employed to magnetize the YIG (in the direction of light propagation). Linearly polarized light emitted by a laser and transmitted by the polarizer is directed into the YIG material. Under the influence of the net magnetic moment within the (magnetized) material, the linearly polarized light experiences circular birefringence. (In a bulk material, e.g., bulk single crystal YIG, linearly polarized light may be represented as consisting of right- and left-circularly polarized components. Circular birefringence means the two components see different indices of refraction, resulting in one of these components propagating through the material at a faster than the other.) As a consequence, the light remains linearly polarized, but the polarization direction is continuously rotated in either the clockwise or counterclockwise (as viewed in FIG. 3) direction as the light traverses the material. (This phenomenon, commonly referred to as the Faraday Effect or magneto-optical rotation, is described in, for example, the McGraw Hill Encyclopedia on Science and Technology, 5th edition, Vol. 5 (McGraw Hill, 1982), p. 314.) If the material is of appropriate dimension, the light is rotated through, for example, 45 degrees and is thus transmitted by an appropriately oriented analyzer. Reflected light transmitted by the analyzer also enters the YIG material and also undergoes a rotation of 45 degrees in the same direction as the light which originally traversed the material. Consequently, reflected light, after traversing the YIG material, is oriented at 90 degrees to the polarizer, and is thus precluded from impinging upon the laser. (The phenomenon by which a magnetized material rotates both forward and backward propagating linearly polarized light by 45 degrees (or an odd multiple of 45 degrees) in the same direction is denoted antireciprocal magnetooptical rotation. Devices which include such materials are referred to as antireciprocal devices.)
A second type of device based on rotation of linearly polarized light is a circulator. When employed, for example, in an optical communication system, such a device efficiently couples light signals from a semiconductor laser into one end of an optical fiber, and allows detection of counterpropagating light signals emanating from the same fiber end. One type of optical circulator (having a configuration suitable for efficiently coupling light into and out of an optical fiber end) is depicted in FIG. 4. This circulator, like the exemplary isolator, includes bulk single crystal YIG, and also includes a polarization sensitive reflector. In operation, a magnet is used to magnetize the YIG in the direction of light propagation. Linearly polarized light, e.g., horizontally (as viewed in FIG. 4) linearly polarized light, emanating from the optical fiber end, is directed into the magnetized YIG. (The optical fiber is, for example, a polarizing fiber. Alternatively, an appropriately oriented polarizer is positioned between a nonpolarizing fiber and the YIG.) If the YIG is of appropriate dimension, the light is rotated through, for example, 45 degrees (in the clockwise direction, as viewed from the fiber in FIG. 4) and is transmitted by the polarization sensitive reflector to a detector. Linearly polarized tight emitted by a laser and oriented at, for example, -45 degrees (relative to the linearly polarized light emanating from the fiber) is reflected by the polarization sensitive reflector into the magnetized YIG. After propagating through the YIG, this light has been rotated 45 degrees (in the clockwise direction, as viewed from the fiber in FIG. 4), and thus enters the fiber horizontally linearly polarized.
While antireciprocal, light rotating devices based on bulk materials, e.g., single crystal YIG isolators and circulators, are useful, they are bulky (have typical dimensions of 3 mm by 3 mm by 3 mm), require the application of large magnetic fields (typically larger than about 1000 oersteds (Oe)), are expensive (typically costing about 1000 dollars), and are thus not entirely commercially attractive. By contrasts, a thin (having a thickness less than about 10 times the wavelength of the incident light) film waveguide antireciprocal device, e.g., a thin film optical isolator or circulator, using planar magnetization would be a much more attractive device. For example, a thin film device would permit the use of guided wave optics (and thus eliminate the need for focusing lenses, not shown in FIGS. 1-4), require the application of relatively small magnetic fields (smaller than about 100 Oe), and be relatively inexpensive. In addition, it could also serve as a building block for integrated optical devices (an optical device which includes two or more components, performing different functions, and formed on the same substrate) useful in optical systems.
Thin film waveguiding devices employing planar magnetization have, in fact, been fabricated. Such devices have included, for example, a magnetized (in the plane of the film) layer of YIG epitaxially grown on a (closely lattice matched) substrate of, for example, gadolinium gallium garnet (Gd.sub.3 Ga.sub.5 O.sub.12, called GGG), While these devices are potentially attractive, they are, unfortunately, subject to linear birefringence. That is, in a thin film, linearly polarized light may be represented as consisting of two orthogonal, linearly polarized components. In one of these components, the electric field of the light (an electromagnetic wave) is oriented parallel to the film surface and is denoted the TE component. In the other component, the electric field is oriented perpendicularly to the film surface is denoted the TM component. Linear birefringence means that the TE and TM components see different refractive indices, resulting in one of these components propagating through the film at a faster speed than the other. (Regarding linear birefringence in thin film waveguides see, e.g., P. K. Tien, App. Opt., Vol. 10, p. 2395 (1971).) Thus, when traversing a magnetized thin film, e.g., a magnetized layer of YIG, light is subjected to elliptic birefringence, i.e., a birefringence which includes both a linear component and a circular component. As a consequence, initially linearly polarized light undergoes oscillatory rotation. (The distance traversed by the light in completing one oscillation is called the birefringent period, P.) This oscillation is depicted in FIG. 5 where the incident light impinges upon a magnetized thin film at an angle of, for example, 0 degrees (to the y-axis). While propagating through the film, the light is initially rotated through a relatively small angle, e.g., 3 degrees, in, for example, the clockwise direction. Further propagation produces a counterrotation to -3 degrees, and still further propagation to a distance P results in the light returning to its initial orientation (i.e., parallel to the y-axis). During this oscillatory rotation, the polarization of the light also varies continuously from linear to elliptic to linear. Because the amplitude of the oscillation is constant and, for most materials, small, e.g., 3 or 4 degrees, little or no net rotation is achieved. But, as discussed, an antireciprocal device must achieve a rotation substantially beyond that normally achieved in linearly birefringent materials, and on exiting, the light should be substantially linearly polarized to avoid, for example, optical power loss at the analyzer of an optical isolator. Thus, the effects of linear birefringence in thin film, magnetized, waveguiding devices have presented a serious obstacle to their advantageous use.
The factors responsible for the linear birefringence found in thin films of, for example, YIG have been identified. One of these factors is what is here termed shape linear birefringence, which is due to the presence of discontinuities in refractive index at the film-air and film-substrate interfaces. These discontinuities affect the TM component differently from the way they affect the TE component, producing an effective refractive index anisotropy in the film. Significantly, the magnitude of the shape linear birefringence increases as the thickness of the film is decreased.
A second factor responsible for linear birefringence, commonly termed stress-induced linear birefringence, is due to a lattice mismatch between the film and the substrate. This mismatch subjects the film to either a compressive or tensile stress in the plane of the film, which also has the effect of inducing a refractive index anistropy in the film. Generally, the magnitude of the stress-induced linear birefringence is independent of film thickness.
Yet a third factor responsible for linear birefringence, commonly termed growth-induced linear birefringence, is due to a non-uniform distribution of certain ions in the film crystal lattice, produced by the conventional techniques used to epitaxially grow films on substrates. That is, in the case of, for example, YIG films, the Y (yttrium) cation is often partially replaced with one or more different cations, e.g., Bi or Nd, to alter the properties of the films. When such a film is grown on a substrate using conventional techniques, e.g., liquid phase epitaxy (LPE), the resulting distribution of the replacement cations in the direction perpendicular to the plane of the film is often different from the corresponding distribution in the plane of the film. This difference in the cation distributions also gives rise to a refractive index anisotropy. As with the stress-induced linear birefringence, the magnitude of the growth-induced linear birefringence is generally independent of film thickness.
It is known that, in many cases, the sign of the stress-induced and/or growth-induced linear birefringence is opposte to that of the shape linear birefringence. Thus, it has been suggested that these different sources of linear birefringence be used to cancel each other to produce zero net linear birefringence.
Different attempts have been made to implement the above suggestion, and different parameters have been measured to judge the efficacy of the attempts. To permit meaningful comparisons, these attempts are described below with reference to a single, nondimensional parameter .alpha. (readily inferable from the different measured parameters), where ##EQU1## Here, .DELTA..beta.=2.pi..multidot..DELTA.n/.lambda., where .DELTA.n denotes the difference in the refractive indices seen by the TE and TM components, while .lambda. denotes the wavelength of the light. Physically, .DELTA..beta. is the phase difference (induced by the net linear birefringence) between the TE and TM components per unit length of film, and has dimensions of, for example, radians per centimeter. In addition, K denotes the Faraday rotation per unit length of the film, and has identical units to that of .DELTA..beta.. Relatively low values of .alpha., i.e., values of .alpha. less than or equal to about 0.1, are desirable and imply high efficacy, while values of .alpha. greater than about 0.1 are undesirable and imply low efficacy.
In one attempt to achieve zero net linear birefringence, Nd-doped YIG films were grown on a GGG substrate, the amount of Nd being controlled to control lattice mismatch and thus stress-induced linear birefringence. (See T. Okuda et al, "LPE Growth of YNd-Iron Garnet Films For Magnetooptical Waveguides," Journal of Magnetism and Magnetic Materials, Vol. 35 (1983), pp. 164-166.) To determine the effects of film thickness, these films were etched to smaller and smaller thicknesses, and the linear birefringence properties of the films, both prior and subsequent to etching, were measured. Significantly, the unetched films had values of .alpha. as low as, but no lower than, about 0.5. After etching, the films exhibited substantially higher values of .alpha., with .alpha. increasing as film thickness decreased.
In a second attempt to produce zero net linear birefringence, (BiGdLu).sub.3 (FeGa).sub.5 O.sub.12 films were grown on a GGG substrate, and annealed at different temperatures, for different amounts of time. (See K. Ando et al, "Annealing effects on growth-induced optical birefringence in liquid-phase-epitaxial-grown Bi-substituted iron garnet films, " Journal of Applied Physics, Vol. 57, No. 4, 15 Feb. 1985, pp. 1277-1281). It was found that annealing serves to reduce growth-induced linear birefringence and, in this instance, leads to reduced values of .alpha.. However, the lowest measured value of .alpha. was no lower than 0.14.
Significantly, at least one group of authors (who are presumably familiar with the above attempts) has concluded that tolerances in the manufacturing processes make it impossible to achieve sufficient control over the different sources of linear birefringence to produce films having acceptably low, net linear birefringences. (See H. Dammann et al, "Phase Matching in Symmetrical Single-mode Magneto-optic Waveguides by Application of Stress," Applied Physics Letters, Vol. 29, No. 26, 29 Dec. 1986, pp. 1755-1757). Rather, these authors have concluded that the net linear birefringences exhibited by conventionally manufactured films can only be reduced to acceptably low values through the application of an external stress as applied, for example, via a pneumatic table.
Thus, those engaged in the development of optical systems employing thin film, waveguiding, polarization-rotating devices have sought, thus far without success, convenient techniques for reducing the net linear birefringences in these devices to acceptably low levels.